SIMOX or separation by implantation of oxygen is a method of fabricating a silicon-on-insulator (SOI) material which can be used in the manufacturing of integrated circuits. SIMOX involves using high-energy ions to implant a large dose of oxygen ions beneath the surface of a bulk Si wafer. Upon high-temperature annealing, the implanted oxygen forms a continuous buried oxide which electrically isolates the Si at the surface (i.e. the superficial layer). Typically, this method has been used to fabricate SOI with a superficial Si and oxide layer thickness of several thousand angstroms.
The implantation of oxygen requires very large doses of O+ ions such as 1.5 to 2.0.times.10.sup.18 cm.sup.-2 which takes an hour per wafer in the state of the art ion SIMOX implanter which is expensive. There is an economic incentive to push to thinner buried oxide (BOX) layers to reduce the time required to implant oxygen. Thinner layers are also consistent with the recent development within the microelectronics industry in which low power, portable electronics has become increasingly in demand. Efforts are now underway in industry to produce a SOI material with a buried oxide layer thickness less than or equal to 1000 Angstroms. However, there are difficulties in fabricating such thin buried oxides by the SIMOX method. The reason for this is that since the implanted oxygen is far in excess of its solubility in Si, oxygen comes out of solution as oxide precipitates. At higher doses of oxygen, the precipitates coalesce to form a continuous oxide layer. However, at the smaller doses used to form the thin film SOI, these precipitates remain isolated. The precipitates can be seen in cross-sectional, transmission electron micrographs (XTEM) after high temperature annealing at greater than or equal to 1300.degree. C. where the implanted oxygen at a dose of 3.times.10.sup.17 cm .sup.-2 is seen to form isolated islands or pipes of oxide beneath the surface as shown in FIG. 10. Isolated islands or pipes of oxide are unacceptable since they do not provide any isolation for the superficial layer of Si.
In the oxygen dose regime from 3.0 to 4.5.times.10.sup.17 cm.sup.-2, a mixture of Si islands submerged in a BOX layer are created. At a dose of about 4.times.10.sup.17 cm.sup.-2, the Si island density is minimum in a continuous BOX layer as described by Nakashima et al., SOI Proceeding, pp. 358-367, (1992), Electrochemical society, Pennington, N.J. However for a dose below 4.times.10.sup.17 cm.sup.-2 at energies greater than or equal to 170 kev, the BOX layer is found to be discontinuous.
In order to improve the SIMOX process, several different post-implantation annealing conditions have been proposed by D. K. Sadana et al. in publications in Mat. Res. Soc. Symp. Proc. 316, 699 (1994) and Proc. 1993 IEEE International SOI Conference, IEEE, Piscataway, N.J. 1993, p.16. A promising improvement involves annealing the implanted wafers in an oxidizing environment which is described by S. Nakashima et al., Proc. 1994 IEEE International SOI conference, IEEE, Piscataway, N.J. p.71. In addition to oxide growth at the surface, oxidation also occurs at internal interface(s) of the buried oxide. Even though the rate of internal oxidation is much slower, it is enough to improve the continuity of the thin buried oxide (BOX). However, it is noted that with this technique, a substantial portion of the superficial Si is consumed by oxidation. This consummation of Si obviously limits the SIMOX process to SOI prepared using relatively high-energy oxygen ions to produce a sufficiently thick layer of Si at the wafer surface to serve as a sacrificial layer.
Improvements to the SIMOX process have been proposed by manipulation of the implantation conditions to promote coalescence of the implanted oxygen. A variety of implantation processes for forming thin, continuous buried oxides using SIMOX has been published by F. Namavar et al., Mat. Res. Soc. Symp. Proc. 235, 109 (1992) and by Y. Li et al., Mat. Res. Soc. Symp. Proc. 235, 115 (1992). All of these processes involve the use of much lower energy ions, equal to or less than 40 keV, than the current process which uses ions of an energy equal to or greater than 150 keV. As such, these processes using ions of energy less than 40 keV are not commercially viable because ion implanters do not operate efficiently in the 40 keV energy regime, so that any cost benefit of implanting a lower dose is nullified by this lost efficiency (i.e. beam current).